Transient voltage suppression device

ABSTRACT

A bi-directional transient voltage suppression (“TVS”) device ( 101 ) includes a semiconductor die ( 201 ) that has a first avalanche diode ( 103 ) in series with a first rectifier diode ( 104 ) connected cathode to cathode, electrically coupled in an anti-parallel configuration with a second avalanche diode ( 105 ) in series with a second rectifier diode ( 106 ) also connected cathode to cathode. All the diodes of the TVS device are on a single semiconductor substrate ( 301 ). The die has a low resistivity buried diffused layer ( 303 ) having a first conductivity type disposed between a semiconductor substrate ( 301 ) having the opposite conductivity type and a high resistivity epitaxial layer ( 305 ) having the first conductivity type. The buried diffused layer shunts most of a transient current away from a portion of the epitaxial layer between the first avalanche diode and the first rectifier diode, thereby reducing the clamping voltage relative to the breakdown voltage. The TVS device is packaged as a flip chip ( 202 ) that has four solder bump pads ( 211 - 214 ). The abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims pursuant to 37 C.F.R. §1.72( b ).

RELATED APPLICATION

This is a divisional application of U.S. patent application Ser. No.11/008,416, filed Dec. 9, 2004 now U.S. Pat. No. 7,361,942, which is acontinuation application of U.S. patent application Ser. No. 10/635,088,filed Aug. 5, 2003 now U.S. Pat. No. 6,867,436, hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to active solid-state devices, and moreparticularly to a transient voltage suppression device having one ormore avalanche diodes.

2. Description of the Related Art

Transient voltage suppression (“TVS”) devices comprising an avalanchediode are well known. As the reverse avalanche voltage is made smaller,a depletion region of the avalanche diode narrows, resulting in a higherinternal capacitance of the avalanche diode. As operating frequenciesbecome higher, the internal capacitance of the avalanche diode becomesproblematic. A known solution to the capacitance problem is to add arectifier diode in series with the avalanche diode, with either theanodes or the cathodes of the diodes connected together. A rectifierdiode has a smaller capacitance than an avalanche diode has, and thetotal capacitance of a pair of such diodes in series is less than thesum of the two capacitances.

TVS devices having both diodes of such pair on a single die are alsoknown. For example, U.S. Pat. No. 6,392,266 entitled TRANSIENTSUPPRESSING DEVICE AND METHOD, issued May 21, 2002, to Robb et al.,discloses two transient voltage suppressors that are housed in a singlesemiconductor package, each transient voltage suppressor comprising twoserially coupled diodes on one die. A TVS device comprising such a pairof diodes is a unidirectional device, in that the TVS device providesprotection against voltage spikes or surges in one direction only.

Bi-directional TVS devices comprising two such pairs of diodes in ananti-parallel configuration are also known. Known bi-directional TVSdevices comprise at least two die, wire bonded together inside a singlesemiconductor package. One example of such a TVS device is the Model No.PSLCO3 through PSLC24C family of TVS devices manufactured by ProTekDevices of Phoenix, Ariz., which includes four die inside a singlesemiconductor package. Such TVS devices work well for their intendeduses, but when a very small bi-directional TVS device is required, a TVSdevice comprising a single die is preferred.

The reverse avalanche voltage, or breakdown voltage, is defined as thevoltage at which the avalanche diode goes into avalanche mode, measuredat a relatively low current such as one milliamp. The breakdown voltageis controlled by the doping level of an N+ diffusion layer relative tothe doping level of a P+ diffusion layer of the avalanche diode. Theclamping voltage is defined as the maximum voltage across the TVS devicewhen a maximum surge current is flowing through it. The clamping voltageis typically measured at a relatively high current such as one amp. As aresult, the clamping voltage is normally higher than the breakdownvoltage. The clamping voltage of a TVS device is directly, although notnecessarily linearly, proportional to the breakdown voltage of theavalanche diode. The amount by which the clamping voltage is greaterthan the breakdown voltage is directly proportional to the geometry ofthe PN junction and to the diffusion depth of the avalanche diode. Ahigher background resistivity of a doped epitaxial region of the dieresults in a higher clamping voltage relative to the breakdown voltage.

As electronic devices, especially battery-operated portable electronicdevices such as cellular telephones become smaller, there is a need fora smaller TVS device. It is desirable that a TVS device has as low aclamping voltage as possible. When the TVS device reaches its clampingvoltage, the TVS device prevents the electronic device under protectionfrom exposure to any higher voltage than the clamping voltage. Theclamping voltage of a prior art TVS device would disadvantageously riseif the avalanche diode were simply made smaller because a smaller PNjunction area has a higher resistance.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide asemiconductor die that overcomes the disadvantages of the prior art, andin particular, to provide a semiconductor die that has a low clampingvoltage.

It is another object of the present invention to provide a TVS devicethat overcomes the disadvantages of the prior art, and in particular, toprovide a TVS device that has a low clamping voltage.

It is still another object of the present invention to provide a flipchip that overcomes the disadvantages of the prior art, and inparticular, to provide a flip chip that has a low clamping voltage.

These and other objects of the present invention will become apparent tothose skilled in the art as the description thereof proceeds.

SUMMARY OF THE INVENTION

Briefly described, and in accordance with a preferred embodimentthereof, the present invention relates to a semiconductor die thatincludes a semiconductor substrate diffused with a first material togive the substrate a first conductivity type. The substrate has asubstrate surface. A buried layer is selectively formed in the substratesurface and is diffused with a second material to give the buried layerthe opposite conductivity type as the substrate. An epitaxial layer isformed on the substrate surface and on the buried layer. The epitaxiallayer is of the opposite conductivity type as the substrate. Theepitaxial layer has an epitaxial surface distal from the substratesurface. A first diffused region is selectively formed on the epitaxialsurface. The first diffused region is of the same conductivity type asthe epitaxial layer. The first diffused region has a first surfacedistal from the substrate surface. A second diffused region isselectively formed on the first surface. The second diffused region isof the opposite conductivity type as the first diffused region. Thefirst diffused region and the second diffused region combine to form afirst semiconductor junction. A third diffused region is selectivelyformed on the epitaxial surface remote from the first diffused region.The third diffused region is of the opposite conductivity type as theepitaxial layer. The epitaxial layer and the third diffused regioncombine to form a second semiconductor junction.

Another aspect of the present invention relates to a transient voltagesuppression device, which includes a semiconductor die that includes asemiconductor substrate diffused with a first material to give thesubstrate a first conductivity type. The substrate has a substratesurface. A buried layer is selectively formed in the substrate surfaceand is diffused with a second material to give the buried layer theopposite conductivity type as the substrate. An epitaxial layer isformed on the substrate surface and on the buried layer. The epitaxiallayer is of the opposite conductivity type as the substrate. Theepitaxial layer has an epitaxial surface distal from the substratesurface. A first diffused region is selectively formed on the epitaxialsurface. The first diffused region is of the same conductivity type asthe epitaxial layer. The first diffused region has a first surfacedistal from the substrate surface. A second diffused region isselectively formed on the first surface. The second diffused region isof the opposite conductivity type as the first diffused region. Thefirst diffused region and the second diffused region combine to form afirst semiconductor junction. A third diffused region is selectivelyformed on the epitaxial surface remote from the first diffused region.The third diffused region is of the opposite conductivity type as theepitaxial layer. The epitaxial layer and the third diffused regioncombine to form a second semiconductor junction.

A further aspect of the present invention relates to a flip chip thatincludes a transient voltage suppression device, which includes asemiconductor die that includes a semiconductor substrate diffused witha first material to give the substrate a first conductivity type. Thesubstrate has a substrate surface. A buried layer is selectively formedin the substrate surface and is diffused with a second material to givethe buried layer the opposite conductivity type as the substrate. Anepitaxial layer is formed on the substrate surface and on the buriedlayer. The epitaxial layer is of the opposite conductivity type as thesubstrate. The epitaxial layer has an epitaxial surface distal from thesubstrate surface. A first diffused region is selectively formed on theepitaxial surface. The first diffused region is of the same conductivitytype as the epitaxial layer. The first diffused region has a firstsurface distal from the substrate surface. A second diffused region isselectively formed on the first surface. The second diffused region isof the opposite conductivity type as the first diffused region. Thefirst diffused region and the second diffused region combine to form afirst semiconductor junction. A third diffused region is selectivelyformed on the epitaxial surface remote from the first diffused region.The third diffused region is of the opposite conductivity type as theepitaxial layer. The epitaxial layer and the third diffused regioncombine to form a second semiconductor junction.

Yet another aspect of the present invention relates to a bi-directionaltransient voltage suppression device, formed on one monolithicsemiconductor die, which includes a first single-directional, transientvoltage suppression circuit electrically coupled to a secondsingle-directional, transient voltage suppression circuit in ananti-parallel configuration. The first single-directional, transientvoltage suppression circuit includes a P+ substrate that has a substratesurface. A first N+ buried layer is selectively formed in the substratesurface. An N+ epitaxial layer is formed on the substrate surface and onthe first N+ buried layer. The N+ epitaxial layer has an epitaxialsurface distal from the substrate surface. An N+ first diffused regionis selectively formed on the epitaxial surface and has a first surfacedistal from the substrate surface. A P+ second diffused region isselectively formed on the first surface of the N+ first diffused region.The N+ first diffused region and the P+ second diffused region combineto form a first semiconductor junction. A P+ third diffused region isselectively formed on the epitaxial surface remote from the N+ firstdiffused region. The N+ epitaxial layer and the P+ third diffused regioncombine to form a second semiconductor junction. The secondsingle-directional, transient voltage suppression circuit includes theP+ substrate, a second N+buried layer selectively formed in thesubstrate surface, and the N+ epitaxial layer. An N+ fourth diffusedregion is selectively formed on the epitaxial surface and has a fourthsurface distal from the substrate surface. A P+ fifth diffused region isselectively formed on the fourth surface of the N+ fourth diffusedregion. The N+ fourth diffused region and the P+ fifth diffused regioncombine to form a third semiconductor junction. A P+ sixth diffusedregion is selectively formed on the epitaxial surface remote from the N+fourth diffused region. The N+ epitaxial layer and the P+ sixth diffusedregion combine to form a fourth semiconductor junction.

Still another aspect of the present invention relates to a method ofmanufacturing a transient voltage suppression device on a single,monolithic semiconductor die that has a top surface. The die has a firstavalanche diode in series with a first rectifier diode, connectedcathode-to-cathode, which is electrically coupled in an anti-parallelconfiguration with a second avalanche diode in series with a secondrectifier diode, also connected cathode-to-cathode. The method includesthe steps of: (a) applying an aluminum metalization layer to the entiretop surface of the die; and (b) removing the aluminum metalization layerexcept for selected portions thereof to form a first aluminum region atone end of the die and a second aluminum region an opposite end of thedie, such that the first aluminum region electrically couples the anodeof the first avalanche diode to the anode of the second rectifier diode,and such that the second aluminum region electrically couples the anodeof the first rectifier diode to the anode of the second avalanche diode,thereby forming a bi-directional transient voltage suppression devicefor providing protection against voltage spikes or surges, in twodirections.

Still another aspect of the present invention relates to a method ofmanufacturing a flip chip on a single, monolithic semiconductor die thathas a top surface. The die has a first avalanche diode in series with afirst rectifier diode, connected cathode-to-cathode, which iselectrically coupled in an anti-parallel configuration with a secondavalanche diode in series with a second rectifier diode, also connectedcathode-to-cathode. The method includes the steps of: (a) applying analuminum metalization layer to the entire top surface of the die; (b)removing the aluminum metalization layer except for selected portionsthereof to form a first aluminum region at one end of the die and asecond aluminum region an opposite end of the die, such that the firstaluminum region electrically couples the anode of the first avalanchediode to the anode of the second rectifier diode, and such that thesecond aluminum region electrically couples the anode of the firstrectifier diode to the anode of the second avalanche diode; (c) applyinga passivation layer to the entire top surface of the die; and (d)removing selected portions of the passivation layer such that a firstsolder bump pad and a second solder bump pad are opened over the firstaluminum region, and such that a third solder bump pad and a fourthsolder bump pad are opened over the second aluminum region.

Other aspects, features and advantages of the present invention willbecome apparent to those skilled in the art from the following detaileddescription and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described with greater specificity andclarity with reference to the following drawings, in which:

FIG. 1 is a schematic electrical diagram of a TVS device in accordancewith the invention;

FIG. 2 is a simplified plan view of a semiconductor die of the TVSdevice in accordance with the invention;

FIG. 3 is a simplified cross-sectional view of the semiconductor die ofFIG. 2 through cut line 3-3;

FIG. 4 is a simplified right side view of the semiconductor die of FIG.2;

FIG. 5 is a simplified cross-sectional view of an alternate embodimentof the semiconductor die of FIG. 2 through cut line 3-3; and

FIGS. 6-12 are simplified representations of masks used to manufacturethe TVS device in accordance with the invention.

For simplicity and clarity of illustration, the drawing figuresillustrate the general manner of construction, and descriptions anddetails of well-known features and techniques are omitted to avoidunnecessarily obscuring the invention. Furthermore, elements in thedrawing figures are not necessarily drawn to scale.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiments discussed below are only examples of the manyadvantageous uses of the innovative teachings herein. In general,statements made in the specification of the present application do notnecessarily limit any of the various claimed inventions. Moreover, somestatements may apply to some inventive features but not to others. Ingeneral, unless otherwise indicated, singular elements may be in theplural and vice versa with no loss of generality, e.g., one die, twodie. The terms first, second, and the like, in the description and inthe claims, if any, are used for distinguishing between similar elementsand not necessarily for describing a sequential or chronological order.The terms top, front, side, and the like, in the description and in theclaims, if any, are used for descriptive purposes and not necessarilyfor describing relative positions.

FIG. 1 is a schematic electrical diagram of a TVS device 101 inaccordance with the invention. The TVS device 101 comprises a firstavalanche diode 103 in series with a first rectifier diode 104 connectedcathode to cathode (“first pair”), electrically coupled in ananti-parallel configuration with a second avalanche diode 105 in serieswith a second rectifier diode 106 also connected cathode to cathode(“second pair”). The anode of the first avalanche diode 103 and theanode of the second rectifier diode 106 are connected to a same firstnode 110. The anode of the first rectifier diode 104 and the anode ofthe second avalanche diode 105 are connected to a same second node 112.In a typical use of the TVS device 101 in a common mode configuration,the first node 110 is connected to a line, and the second node 112 isconnected to a ground. However, the TVS device 101 is symmetrical, andthe first node 110 and the second node 112 are interchangeable.

The TVS device 101 in accordance with the invention is a bi-directionaldevice in that the TVS device provides protection against voltagetransients, spikes and surges in both directions. During normaloperation, voltage swings are lower than the breakdown voltage of theavalanche diodes 103 and 105; therefore, current does not flow throughthe TVS device 101. If a negative transient signal occurs that isgreater than the breakdown voltage, the first avalanche diode 103 breaksdown, thereby routing a surge current, through both the first avalanchediode 103 operating in an avalanche mode and the first rectifier diode104 operating in a forward conducting mode, to ground. Concurrently, thesecond rectifier diode 106 operates in a rectifying mode. If a positivetransient signal occurs that is greater than the breakdown voltage, thesecond avalanche diode 105 breaks down, thereby routing a surge current,through both the second avalanche diode 105 operating in the avalanchemode and the second rectifier diode 106 operating in the forwardconducting mode, to ground. Concurrently, the first rectifier diode 104operates in the rectifying mode. As a result, the voltage is limited tothe clamping voltage of the TVS device 101.

When no transient signal is present, the TVS device 101 produces a loadwith a high impedance at its intended operating frequency, as a resultof the low capacitance of the rectifier diodes 104 and 106. Thecapacitance of the TVS device 101 in accordance with the invention isless than approximately 10 ρF. The intended operating frequency of theTVS device 101 is approximately 500 MHz. The clamping voltage of the TVSdevice 101 is preselected to be in the range of approximately 8-30volts.

FIG. 2 is a simplified plan view of the TVS device 101 in accordancewith the invention. The TVS device 101 comprises one monolithicsemiconductor die, or die, 201. The one die 201 comprises the firstavalanche diode 103 in series with the first rectifier diode 104connected cathode to cathode, electrically coupled in anti-parallelconfiguration with the second avalanche diode 105 in series with thesecond rectifier diode 106 also connected cathode to cathode.Preferably, the TVS device 101 is packaged as a flip chip 202. The flipchip 202 has four solder bump pads 211-214. Solder bump pads 211 and 214are electrically coupled to the first node 110, and solder bump pads 212and 213 are electrically coupled to the second node 112. A solder bump(not shown) is placed at each solder bump pad 211-214. Only one of thesolder bump pads 211 and 214 at the first node 110, and only one of thesolder bump pads 212 and 213 at the second node 112 are required forelectrical operation because the TVS device 101 is a two terminaldevice. However, a solder bump is placed at each of the four solder bumppads 211-214 for mechanical stability.

FIG. 3 is a simplified cross-sectional view of the die 201 through cutline 3-3. The die 201 comprises a P+ semiconductor substrate, orsubstrate, 301. The P+ substrate 301 has a substrate surface 302. An N+buried layer 303 is disposed on a portion of the substrate surface 302.An N-type epitaxial (“N-EPI”) layer 305 is grown on the substratesurface 302, including over the portion of the substrate surface havingthe N+ buried layer 303. The N-EPI layer 305 has an epitaxial surface306 distal from the substrate surface. There is a P+ isolation diffusionregion 307 around the perimeter region of the die 201. This perimeterregion prevents an increase in leakage current that might occur as aresult of damage to the edges of the die 201 that usually occurs whenthe die is separated from its wafer. The leakage current might cause thedie to test “bad”, and might cause some undesirable interaction betweenthe first pair and the second pair. A portion 408 (see FIG. 4) of the P+isolation diffusion region 307 extends between the first pair and thesecond pair. The portion 408 isolates the first pair from the secondpair. The portion 408 of the P+ isolation diffusion region 307electrically isolates the two anti-parallel circuits on the die 201, andallows the two anti-parallel circuits to be on one die withoutinterfering with one another.

A heavily doped, N+ first diffused region 311 is disposed on theepitaxial surface 306 of the N-EPI layer 305. The N+ first diffusedregion 311 is the cathode of the first avalanche diode 103. The N+ firstdiffused region 311 has a first surface 312 distal from the substratesurface 302. A P+ second diffused region 313 is disposed on the firstsurface 312 of the N+ first diffused region 311. The P+ second diffusedregion 313 is the anode of the first avalanche diode 103. The P+ seconddiffused region 313 has a second surface 314 distal from the substratesurface 302. The first avalanche diode 103 is formed by a PN, orsemiconductor, junction between the N+ first diffused region 311 and theP+ second diffused region 313 of die 201.

A P+ third diffused region 315 is disposed on the epitaxial surface 306of the N-EPI layer 305. The P+ third diffused region 315 is the anode ofthe first rectifier diode 104. The P+ third diffused region 315 has athird surface 316 distal from the substrate surface 302. The firstrectifier diode 104 is formed by a semiconductor junction between theN-EPI layer 305 and the P+ third diffused region 315 of the die 201. Thesecond avalanche diode 105 and the second rectifier diode 106 of thesecond pair are similar to the first avalanche diode 103 and the firstrectifier diode 104 of the first pair, respectively, and, therefore, arenot described in detail.

During a transient over-voltage event, the N+ buried layer 303 reducesthe clamping voltage of the TVS device 101. The clamping voltage iscontrolled by the geometry and doping of the N+ buried layer 303.Referring back to FIG. 2, the N+ buried layer 303 has a length 215 and awidth 216. The N+ buried layer 303 advantageously reduces the clampingresistance of the TVS device 101 when the TVS device is in an avalanchemode. The clamping voltage of the TVS device 101 in accordance with theinvention is improved, i.e., lowered, by about 20% over prior art TVSdevices. For example, a typical prior art TVS device has a clampingvoltage of 9.5 volts, whereas, the TVS device 101 in accordance with theinvention has a clamping voltage of only 7.6 volts.

The N-EPI layer 305 on the silicon P+ substrate 301 is a highresistivity (approximately 70 ohm-cm) N-type material. The resistivityof the N+ buried layer 303 is an order of magnitude lower than theresistivity of the N-EPI layer 305. The doping levels and the thicknessof the layers obtained through diffusion and the lateral dimensions foreach element are well known to a person skilled in the art ofsemiconductors. The high resistivity of the N-EPI layer 305 is needed toproduce a high reverse voltage for the rectifier diodes 104 and 106. Thehigh reverse voltage for the rectifier diodes 104 and 106 is needed sothat the first rectifier diode 104 of the first pair is not in avalanchemode when the second pair is conducting during a transient event, andvice versa. Referring again to FIG. 3, the N-EPI layer 305 forms a highresistivity conduction path between the first avalanche diode 103 andthe first rectifier diode 104 for conducting the surge current during avoltage surge. A distance 317 between the first avalanche diode 103 andthe first rectifier diode 104 is approximately 150 μm. In a prior artTVS device lacking the N+ buried layer 303, a surge current would travelthe distance 317 between the first avalanche diode 103 and the firstrectifier diode 104 through the high resistivity N-EPI layer 305. In aprior art TVS device, the relatively long distance 317 would cause alarge voltage drop between the first avalanche diode 103 and the firstrectifier diode 104 when the first avalanche diode is in avalanche modeand conduction occurs between the first avalanche diode and the firstrectifier diode. A distance 318 between the first avalanche diode 103and the N+ buried layer 303 is approximately 10 μm. A distance 319between the first rectifier diode 104 and the N+ buried layer 303 isapproximately 15 μm.

With the N+ buried layer 303 in accordance with the invention, the surgecurrent goes through the N+ buried layer 303 rather than going solelythrough the N-EPI layer 305 because of the much lower resistivity of theN+ buried layer relative to the N-EPI layer. The N+ buried layer 303reduces the effective length of the high resistivity conduction path(through the N-EPI layer 305) from about 150 μm to about 25 μm, therebyreducing the resistance seen by the surge current, and consequentlyreducing the clamping voltage relative to the breakdown voltage. The N+buried layer 303 significantly reduces the voltage drop between thefirst avalanche diode diffusion area and the first rectifier diodediffusion area. Advantageously, the N+ buried layer 303 shunts most ofthe surge current away from the portion of the high resistivity N-EPIlayer 305 between the first avalanche diode 103 and the first rectifierdiode 104. The N+ buried layer 303 acts as a low resistance region forconduction between the first avalanche diode 103 and the first rectifierdiode 104. Without the N+ buried layer 303, the clamping voltage ishigher and the possibility of damaging electronics beyond the TVS device101 is greater. Advantageously, the N+ buried layer 303 does notincrease the capacitance of the TVS device 101. The N+ buried layer 303has no effect on the breakdown voltage of the first avalanche diode 103,and, at low currents, has no effect on the forward bias voltage of thefirst rectifier diode 104. The operation of the second pair is similarto the operation of the first pair, and, therefore, is not described indetail.

Although the N+ buried layer 303 advantageously reduces the clampingvoltage relative to the breakdown voltage, the N+ buried layer has noeffect on the breakdown voltage itself. For example, the breakdownvoltage of the avalanche diodes 103 and 105 in accordance with theinvention is approximately seven (7) volts; whereas, the clampingvoltage of the avalanche diodes is advantageously only slightly higherat approximately eight (8) volts.

FIG. 4 is a simplified right side view of the die 201.

FIG. 5 is a simplified cross-sectional view of an alternate embodiment500 of the die 201 through cut line 3-3, showing a larger N+ firstdiffused region 502 of the first avalanche diode 103. The larger N+first diffused region 502 extends to the N+ buried layer 303. Althoughthe alternative embodiment 500 takes longer to manufacture because amuch longer N+ diffusion time is required, the advantage of thealternate embodiment is a further reduction of the clamping voltagerelative to the breakdown voltage.

FIGS. 6-12 are simplified representations of masks 600-1200 used tomanufacture the TVS device 101 in accordance with the invention. Amethod of manufacturing a small, low capacitance flip chip 202 that hasbi-directional transient voltage protection and a low clamping voltage,comprises the following steps:

(1) Start with the P+ substrate 301. It should be noted that the P+substrate 301 is semiconductor material with a very high doping levelfor reduced resistivity, and is different from the P-type material ofthe PN junction.

(2) Grow a thermally deposited diffusion oxide, preferably SiO₂, on theP+ substrate 301 and pattern the oxide in the shape of mask A 600, toopen two windows 601 and 602 for N+ diffusion. The larger the area ofthe two windows 601 and 602 in mask A 600, the lower is the resistanceof the two avalanche diodes 103 and 105.

(3) Perform N+ diffusion to a depth 320 of approximately five (5) μm atthe portions of the P+ substrate exposed by the two windows 601 and 602.Upon completion of this N+ diffusion, the N+ buried layer 303 inaccordance with the invention is formed. The greater the depth of the N+buried layer 303, the lower is the resistance. The higher the dopinglevel of the N+ buried layer 303, the lower is the resistance. Thedoping level is controlled by temperature, diffusion time andconcentration of dopant on the surface.

(4) Remove the thermally grown diffusion oxide that was grown in steptwo.

(5) Grow a high resistivity N-EPI layer 305 of approximately 25 μmthickness on the same side of the P+ substrate 301 that was subjected tothe N+ diffusion of step two.

(6) Grow a diffusion oxide 325 on the N-EPI layer 305 and pattern theoxide in the shape of mask B 700 for P+ diffusion.

(7) Perform P+ diffusion on the portions of the N-EPI layer 305 exposedby mask B, such that the diffusion penetrates to the P+ substrate 301.Upon completion of the diffusion, these portions become the P+ isolationdiffusion region 307 and 408.

(8) Apply mask C over the existing diffusion oxide. Mask C 800 has twowindows 801 and 802. The larger the area of these windows 801 and 802,the greater is the current-carrying capability of the resultingavalanche diodes 103 and 105.

(9) Diffuse the N+ regions in the N-EPI layer 305 to form the N+ firstdiffused region 311 of the TVS device 101. The N+ first diffused region311 is used to fix the breakdown voltage of the TVS device 101. Thedepth of the N+ first diffused region 311 is selected to produce apreselected breakdown voltage. A greater depth results in a higherbreakdown voltage, which, in turn, results in a higher clamping voltage.At the same time, re-grow the thermally deposited diffusion oxide 325and apply mask D 900 over the diffusion oxide. Mask D 900 has fourwindows 901-904.

(10) Diffuse the P+ second diffused region 313 and P+ third diffusedregion 315 in both the N+ first diffused region 311 and in a region inthe N-EPI layer 305 over the N+ buried layer 303 and adjacent to, butnot in contact with, the N+ first diffused region. The P+ diffusion ofthis step is selected such that the breakdown voltage will be controlledto a given specification in the N+ first diffused region 311. The P+third diffused region 315 (anode) on the N-EPI layer and the N-EPI layer305 (cathode) form the first rectifier diode junction. The highresistivity of the N-EPI layer 305 and the small size of the junction ofthe first rectifier diode 104 are preselected to provide a specific lowvalue of junction capacitance. The P+ second diffused region 313 (anode)on the N+ first diffused region 311, and the diffused N+ first region(cathode) form a first avalanche diode junction. The high doping level(and low resistivity) of the N+ first diffused region 311, which isrequired for the desired avalanche breakdown voltage, results in a highinternal capacitance of the first avalanche diode 103. At the same time,re-grow the thermally deposited diffusion oxide 325 and apply mask E1000 over the oxide. Mask E 1000 has four windows 1001-1004.

(11) Apply an aluminum metalization layer to the entire top surface ofthe die 201 distal from the substrate surface 302 to provide a firstexternal electrical contact to the second surface 314 of the P+ seconddiffused region 313, and a second external electrical contact to thethird surface 316 of the P+ third diffused region 315, where exposed bywindows 1001-1004 of mask E 1000.

(12) Using mask F 1100, remove the aluminum metalization layer exceptfor portions 1101 and 1102 to form first and second aluminum regions 219and 220 (see FIG. 2) at each end of the die 201, which electricallycouple the anode of the first avalanche diode 103 to the anode of thesecond rectifier diode 106 and the anode of the first rectifier diode104 to the anode of the second avalanche diode 105, respectively. Thealuminum region 219 also electrically couples the external electricalcontact at the second surface 314 to solder bump pads 211 and 214. Thealuminum region 220 also electrically couples the external electricalcontact at the third surface 316 to solder bump pads 212 and 213.

(13) Apply a low temperature, chemically vapor deposited (“CVD”) SiO₂layer 333 over the entire surface of the die 201 as a passivation layer.Alternatively, a nitride or another oxide is used as the passivationlayer.

(14) Apply mask G 1200, and pattern the CVD SiO₂ layer 333 such that twowindows 1201-1202 and 1203-1204 are opened in the CVD SiO₂ layer to thealuminum metalization layer at each end of the die 201.

(15) Apply underbump metallurgy in the open windows comprising nickelwith a flash of gold as a passivant on the nickel surface.

(16) Screen print a solder paste over the underbump metallurgy andreflow the solder to construct the solder bumps.

The description of the method of manufacturing refers primarily to thefirst avalanche diode 103 and the first rectifier diode 104, i.e., thefirst pair, for succinctness; however, the description also applies tothe second pair.

The TVS device 101 and the flip chip 202 have the small dimensions of0.02 inch width by 0.04 inch length by 0.02 inch height, and have a lowclamping voltage of 8-30 volts and a low capacitance of 10 ρF or less.The clamping voltage and the capacitance of the TVS device 101 and theflip chip 202 are improvements over prior art TVS devices and flip chipsof similar physical dimensions. Known prior art TVS devices and flipchips of similar physical dimensions have a clamping voltage of 9-36volts and a capacitance of 30 ρF.

While the present invention has been described with respect to preferredembodiments thereof, such description is for illustrative purposes only,and is not to be construed as limiting the scope of the invention.Various modifications and changes may be made to the describedembodiments by those skilled in the art without departing from the truespirit and scope of the invention as defined by the appended claims.

LIST OF REFERENCE NUMERALS 101 Transient Voltage Suppression (“TVS”)Device 103 First Avalanche Diode 104 First Rectifier Diode 105 SecondAvalanche Diode 106 Second Rectifier Diode 110 First Node 112 SecondNode 201 Semiconductor Die, or Die 202 Flip Chip 211-214 Solder BumpPads 215 Length 216 Width 219-220 Aluminum Regions 301 P+ SemiconductorSubstrate, or Substrate 302 Substrate Surface 303 N+ Buried Layer 305N-Type Epitaxial (“N-EPI”) Layer 306 Epitaxial Surface 307 P+ IsolationDiffusion Region 311 N+ First Diffused Region 312 First Surface 313 P+Second Diffused Region 314 Second Surface 315 P+ Third Diffused Region316 Third Surface 317 Distance 318 Distance 319 Distance 320 Depth 325Thermally Deposited Diffusion Oxide 333 Chemically Vapor Deposited(“CVD”) SiO₂ Layer 408 Portion of the P+ Isolation Diffusion Region 500Alternate Embodiment of the TVS Device 502 Larger N+ First DiffusedRegion 600 Mask A 601-602 Windows 700 Mask B 800 Mask C 801-802 Windows900 Mask D 901-904 Windows 1000 Mask E 1001-1004 Windows 1100 Mask F1101-1102 Portions 1200 Mask G 1201-1204 Windows

1. A method of manufacturing a transient voltage suppression device on asingle, monolithic semiconductor die, the die having a first avalanchediode in series with a first rectifier diode connectedcathode-to-cathode, electrically coupled in an anti-parallelconfiguration with a second avalanche diode in series with a secondrectifier diode connected cathode-to-cathode, the die having a topsurface, comprising the steps of: (a) applying an aluminum metalizationlayer to the entire top surface of the die; and (b) removing thealuminum metalization layer except for selected portions thereof to forma first aluminum region at one end of the die and a second aluminumregion an opposite end of the die, such that the first aluminum regionelectrically couples the anode of the first avalanche diode to the anodeof the second rectifier diode, and such that the second aluminum regionelectrically couples the anode of the first rectifier diode to the anodeof the second avalanche diode, thereby forming a bi-directionaltransient voltage suppression device for providing protection againstvoltage spikes or surges, in two directions.
 2. A method ofmanufacturing a flip chip on a single, monolithic semiconductor die, thedie having a first avalanche diode in series with a first rectifierdiode connected cathode-to-cathode, electrically coupled in ananti-parallel configuration with a second avalanche diode in series witha second rectifier diode connected cathode-to-cathode, the die having atop surface, comprising the steps of: (a) applying an aluminummetalization layer to the entire top surface of the die; (b) removingthe aluminum metalization layer except for selected portions thereof toform a first aluminum region at one end of the die and a second aluminumregion an opposite end of the die, such that the first aluminum regionelectrically couples the anode of the first avalanche diode to the anodeof the second rectifier diode, and such that the second aluminum regionelectrically couples the anode of the first rectifier diode to the anodeof the second avalanche diode; (c) applying a passivation layer to theentire top surface of the die; and (d) removing selected portions of thepassivation layer such that a first solder bump pad and a second solderbump pad are opened over the first aluminum region, and such that athird solder bump pad and a fourth solder bump pad are opened over thesecond aluminum region.